Sound recording



April 26,1932 H. McDov/ELL, JR 1,855,197

` soUD RECORDING V Y Filed Nov. 8. 1930 2 Sheets-Sheet l INVENTOR HupH MG powELL JR.

,. ATTORNEY April 26, 1932.

H, MC DOWELL, JR

SOUND RECORDING 2 sheets-shea 2 Filed Nov. 8, 1930 man Ms DoweLL JB.' BY V d vATTORNEY Patented'Apr. 26;.1932

' UNITED' STATES-PATENT GFFwE nnen ncDowELL. JB., 0I Los ANGnLlgis, carni-omen, samen-)n To: nAnIo-monrnnun oonronarron, a .conron'rIoN or m'zLAND RElssuED s scum) RECORDING hemmen man november' e, 1930. serial no. 494,218.

The present vc ording and sound reproducing systems and films, such as motion picture Iilm recording Land reproducing processes v P must be good, the range of sound ords are 1n methods, and has particular relation to suchsystems and methods whereinthe sound recl the formofphotographic variable area or variable density stri 's upon linear m s,etc.' Sound must meet three essential requirements; reduced rom soft to loud must be considerable, and there mustv be-a diminution' f ground noise when the namely, the quality of the sound re s volume of sound reproduced is low. In the present standard methods of recording, the

first requirement is fulfilled. The second requirement is partiallyffuliilled, 'although the range of volume obtainable is notas great as could be desired.- The third requirement 1s not met, due to what is known asground noise. This, as will-tbe explained more'in detail immediately hereafter, is .the chief fac. tor in limiting the range obtainable in fulfillment of the second requirement set forth above. f z p When a sound record ofthe type-described 'fis moved between va fixed light source -and a photo-'sensitive cell, dust,foil, and dirt ad'v hering to the transparent variations thereof, .as well as the inhere t granular structure of the vfilm, give rise to liuctuation in the intensit'y of light reaching the '"photoelectric' cell and a subsequent production 01E- an elec'-v tric current variation affecting the reproduc ers and'causing them to produce what is commonly known as ground nolse'.

In accordance with the usual process, the

sound is originally recorded upon speed motion picture iilm, thegrain of which is neces- Vsarily large. The portions 'of the negative record which aeblackened arenot entirely blackened as there are 'interstices tween the large grains of the emulsion.. Whenrthe negative record thus produced is used to print the positive, some of the relatively smaller emulsion grains'of the normally clear portions'thereof become exposed through the transparent interstices' in the supposedly opaque portions of the'negative. This granular e'ect, as above noted, adds to the eiec't invention relates to sound re- The intensity of the ground tone is stantially constant throughout any particular portion of the film since the average transparency thereof is relatively constant. .How-- roper handling, whereas the graneen ever, it is much more noticeable and unpleas'- ant during he periods when no sounds or faint sounds are being intentionally reproduced. v

.Tt is obvious, therefore, that with the standard method of recording, the sound v ol` ume reaching the flm'recorder must at 'all times be adequate to overcome the noise factor which, as above noted,remains constant`This -produces a limitation in the range of volume which can be obtained with a given film.

` Accordingly, ain object'of my invention is to. provide asound recording method whereby a photographic Vsound record may he reproduced without an objectionable amount of ground noise.

A further object of my invention is .to pro# duce a recording system which will producea record which maybe reproduced in thev ordinary sound reprodueer and still eliminate' the ground noise to a considerable extent.`

It is a still further object of my invention to produce a recording-process which will result in a record which may be reproduced with t e ordinary vsoun d reproducing equip ment and yet eliminate the ground noise to a.

considerable extent.

It is another object of'myi'invention to pro-1 duce'va recording system which will result in a record which maybe reproduced inthe ordinaryI photophonographic sound reproyducer and permit full volumerangeto be ob tained. 1. .I Y. .j

It is yet another ohject'of my in'venton to reduce a process which will result in arecord which may be Vreproduced .photophono-- graphically and permit full volume range.

. These andfurthe'r objects of my invention specification taken in will become apparent from the following connection with the appendeddrawings. Y*

In accomplishing the objects of my invention, I eliminate the greater part of the clear portion of the'fihn recordr except in so far as it is necessary to permit full modulation at a' given sound intensity.. Thus it is possible to allow the modulation to fall to' its natural minimum and rise to its natural maximum, for withthe 'clear portions 'of the lilm practically eliminated theratio of ground noise. -`to sound remainsfconstant, instead of changing in inverse'proportion to the average' sound intensity.

With the standard'method of sound recording, the film range, for' instance, from minimum to maximum is approximately 2O TUs; while in the antiground noise of my invention, the film range may be made approximately from to 40, TUs.

In carrying out'my invention the usual variable area sound recorder is utilized. This recorder, for instance, may be a galvanometer type utilizing a vibrating mirror actuated by the voice currents. Connected vwith the voice input is an amplifier and a detector for amplifying and detecting thevoice currents respectively. The rectified voice currentsar'e then,

utilized to operate .a shutter placed. the path of the vibrating light beam for the purpose of'limiting the amount of light falling upon the negative sound record to that which is-necessary to permit the full fiuctuation or modulationof the recording light. The shutter will move in proportion to the D. C. impulses representative of the rectifiedY voice current, and will', admit light onlv to the extent required by the peak` modulation reachingthe film at anv instant. When no modulation occurs, theshutter is adjusted to admit light tothe film to the extent of aD- proximately five thousandths .of van inch in width as against the usual thirty-tive thousandths width for the normalrecording system. It is seen by this that there will thereforremain about ten percent of the clear portion of the film' that formerly existed.

-The amount of light reaching the photo cell of the projector, therefore, operated by a positive produced by the negative produced f in accordance with my invention, will be greatly reduced when there is no modulation, and the variation in' light intensity due to dirt, scratches and grain structure,vwill be i correspondinglyl decreased. This in turn greatly reduces the amount of ground noise.

In otherV words, 'the groundnoise will be reduced with the volume, and will maintain a constant ratio relative thereto. This will permit rather slight sounds to be recorded and reproduced, andwill permit the reproducer amplifier to remain set forthe soft portions of the' accompaniment instead of having to be down toprevexit awhisper being/reproduced asa shout.

Having thus brieflyl described my invention, attention isin-vited to the accompanying dravvings,in which;

the .recording of sound in accordance'with "Fig, lis a schematic diagram illustrating.`

my invention, and electrical circuits` for acinl accordance with `my' Figs. 3.. and l illustrate modified embodi- I means of the condenser. amplifier 413, to produce a' current of varying intensity representative of the strength of voice oscillations lized in Which case thev mixer 15 serves to currents as thus combined are supplied by the'mixerl to .theamplifier 17.

is used to actuatethe' vibrating osclllograph mirror 19.v The mirror 19'is the oscillograph mirror ofthe usuaLvariable area sound recording system, schematically illustrated in` this ligure. Thel light from source L is condensed by means of the condenser lens C,- ,passed through the a'perture A1, throu h galvanometer lens GL and :is refiected by he oscillograph mirror 19 again through GL, through the usual optical system O comprising cylindrical lens CL and lens L] which. serveto focus the light in the form of a nar-` row slit Aupon the narrow slot SL, the image combine the currents picked: by the variousmicrophonesin the proper proportion. The

. operating upon the condenser microphone 11'. i 4A plurality of condenser microphone and'v their corresponding modulators mayl be uti- The current produced-by the amplifier-17 '95 of which slot yis focussed in the form of-'a narrow slitof light S upon the sound portion R of the film F.l The film F is moved in such a manner that the narrow slit of light S falls in a fthe motion of said film.

The area ofthe ilm'exposed to the action of the light L is controlled by means of 'the oscillograph mirror 19, and would'in itself produce the usual variable larea record. However, there is interposed between" the oscillograph'mirror`19 and the lens CL anadditional apertured ,shutter A2. The movement f this shutter serves to cut down the width of the sound track in the opposite directiondirection transverse' to from the screening resulting by the combination of the aperture A1 and the oscillograph mirror 19. Thus, referring particularly to the film F, the actiono shutterA2 serves to produce the shaded portion N -of the record R; whereas the conjoint actionof aper- 'l ture A1 and oscillograph mirror 19 serve to produce the shaded-'portion M ofrecord R,

the latter ofcourse being th o ing producediby the ordinary variable area.

i usualr shadsound,recordin`g process. The negativelm produced b thls process will be transparent in the portions indicated by theshadings 1n Fig. 1, the clear portionshaving been exposed f to ,the light, the negative' therefore upon development will appear opaque at such portions, and clear-at the portions which were shaded inthe recording by the aperture A1 Y and mirror 19 or shutter A2. e

In printing from the negativel thus pro.- duced, a positive in the'nature of the .film

' represented in Fig. 2, to which attention is now invited, is produced. AIn this figure, the

Iilm F bears a sound strip record R composed of-v the shaded portions M representative of the voice or other sound waves recorded, and thefshaded portion record N which accom. plishes the. purpose of my invention in reducing the amount of th'e clear portions of saidy positive to the minimum required togive the full light iuctuations when the positive 4record is used to control the light falling upon a photo-sensitive device.

Connected with .the circuit supplying'the;

vibrating mirr'or 19 is a two stage amplifier,l

comprisingthermionic Adevices 21 land 23.'

The output of this ampliier is supplied'to the thermionic rectifier 25. -The' audiocurrent thus rectified is passed to a D. C. am'

plier 27 of ther'monicdevices in parallel.

or the purpose of convenience, a single device 27 has been illustrated. `It is understood, however, that for the single devicev 27 there may bes'ubstituted any desirable number of similar ldevices, arranged in parallel, which latter is the form in which I have oper,- ated my invention.

i The rectified and amplifiedl impulses.

are supplied throughresistande 29, battery 31 and through thevoicei coil 35'of the electrodynamic device3'i' to ground. The Leld 38 tion'edl 'that `when` `no current is {iowing' through the amplifier 27, the current owing.

of device'37 ,is supplied by the battery or f zequivalent.` D. C. potential 40. "Coil is divided 'into'v two portions '33' and 39 oth of which portions serve, when` current vis ment battery 4 1 and through'vavjvariable re-v s1stor'43, the said circuits being so proporthrough the portion 394 of coil'35 will bej sufiif` vcient to move thecoil 3'5 in the opposite direc-v tion.` To' themov'ingcoil 35 of this device,

the; amount olf 'inputpf the amplifier circuit moving outward under'the pressure of the-f forth and rectified cur-rent of the'voifce coils to accomi modate x the l peaks of the modulation, `on the -maiyl-besub'stituted.

" claims. f

film and moving backward, when the modulation has ceased under the pressure of or# tion 39 of the coil 35. The vibration o the aperture'd'shutter A2 is in exact accordance. with .the D. C. impulses of the rectifier and, therefore, it` passes light only to the extent required to permit the peak modula- Wlien no m ulation occurs, the shutter is ad- ]usted to admit a ,beam of light of a width of approximately ve'thousandthsofan inch 'to the recording lilm.

milliammeter 's inserted in lead to portion 33 of voice coil-35 for indicating the variations of current in this system. A similar milliammeter 47 is inserted in the lead from groundv to portion 39 of voice 'coil 35 for a similar purpose.

For rorchestra recording, the recording level may be determined by trying out the vtion reachin the lm a-tany given instant. y i

operation of the recorder on the-highest fortissimo of the orchestra and setting the amplilice-tion so that no over-shooting occurs. Then the amplification havingl once been advjusted, the volume of recording is controlled naturally by the change of volume produced by the. orchestra.

f 'An alternative recording' arrangement may be provided in .which a second mirror GM is substituted'for the shutter A2 of the above described and illustrated arrangement. This second mirror may be operated by the usual galvanomter coils controlled by the otput of the amplifier 27 and `controls the place- -ment of the image of a second light source having its own condenser lens and aperture plate A1. This image is thrown. upon the film Fthrough the 'samekopticahsystem 0 as shown in Fig.

- A second alternative 'device may be ar ranged by controlling the'size of vaperture plate'A, as shown in Fig. 1 and described v above, by'means of a voice coil 33 similar vtothat shown. This is effected by placlng an aperture shutter A2 adjacent the aper- .ture A1 as shown in Fig. 4f. It iisV obvious-that for the electrodynamic shutter motor, 37 ,shown andl described,- an

- electrostatic motor or other secondary mover .which case 'the shutter vaperture A2 nieely'vary the width .of the sound trackref ffcorded. :A -V-shaped aperture .shutter .might Having thusdescribed my invention, atten? i A .tionis invited to the Thus, the shutter movesin proportion to" fact that 1 am not to belimited the 'specific embodiment shown and nils the event of using a variable density systhe system illustrated maybe used y. l

wou

escriben for the purpose of illustration only,

I claim o l. Inl combination, `means for making a photographic sound record within a single sound-track area, and means controlled by the audio currentof the sound being recorded for continuously, altering the character of the record so that, When the original 'sounds are -reproduced therefrom, ground noise is minimized, said means including means responsiveto said current for shading the normally clear portion of the finished sound-track areato the maximum amount possible to still permit obtaining suiiicient ci'earportion to properly reproduce the recorded sound. V

2. In combination, means for' making a photographic sound record Within a single,l

sound-track varea and means, responsiveJ to the audio current of'the sound being record ed'for controlling the average opacity of the sound-track area, said last mentioned means comprising means responsive to said current for shading the normally yclear portion ofv said sound-track areato the limit possible still retaining sufiicient of said clear portion to permit accurate reproduction of recorded sounds. .y i 3. In combination, means` for exposing a light vsensitive surface to light, means responsive to audio current impulses for causing said light toevary in response to sound, and additional means controlled by said impulses for causing the amount of flight reaching said surface to vary independently of the yfrequencies recorded by said first mentioned means.

4.'In combination, `means for exposing a light sensitive surface to light, means responsive to said' audio current impulses for 6. 'In ,'-coinbination, Imeans responsive tof:

audio current impulses for directing anar-v row line of light into a portion of 'film reserved forphotographicV sound record means [6 6 for causingoneend'o'isaid line to `move transj Aversel'y lof said. film in4 response tolsounds; 'and means controlled by said impulses for caus'in'gthe other end of'said line to assume and maintain'an average positionwith ree5: spect' tothe .boundary of said portion correreame? causing said light to vary in'fresponse tov sound, and independent additional meansAv controlled by said impulses for causing the spending to the lou'dness of the sound represented by said record. v 7. The method of making photographic `sound records adapted to' produce a mini/- wmum amount of ground noise upon reproduction which comprises exposing asensitive nlm to a light beam, varying one side of said light beam in accor-dancefwith the Wave form of the sound being recorded, and varying the other. sideof the sa-idbeam in accordance With a rectified component of said Wave.

8. Apparatus for recording sound on amoving llight recordingmember comprisingv means' including a galvanometer responsive to. electrlcal Waves corresponding to sound Waves for exposing said member, means. 1n-

cluding ai secondgalvanoineter `forv varying the exposure thus produced, and means connected in circuit with a second galvanometer for rectif'ying anddamping said electricaLc 9. The method of making varable Width photographic sound records in which 4'the clear area varies in accordance with the volume of the soundbeing recorded, which comprises, exposing ak sensitive film to a single .l

light beam, vibrating one edge of the beam laterally of the film in ,accordance with the wave sound being recorded, and simultane` ously producing\.a lateral displacement of the other ledge on the beam-in accordance vvith a rectified component of said Wave. r

10. Apparatus fo f movinglight sensitive memberwhich coinprises-means for varying the limits of thev vlight falling upon said sensitive member in accordance with sound impulses, said rneans including coil normally excited to move said meansin one direction, and a second coil excited by'said sound impulses to move said means in the other direction, 11. Apparatus for recording sound on a moving light, sensitive 'member which comprises a light source, an aperture, a -galvanometer 'for reilectlng .the light passed through said aperture, an optical system for focussing'the` light thus deflected upon a moving lightlsensitive member inthe form of, a

narrow sl1t transverse thereof, means for ac tuating said galvanoineter in accordance" recording'sound on a -f with sounds in' such a manner that the extent of Villumination of said light sensitive mem? be'r by said beamof light will be defined by" one edge 'of lsaid aperture `and shutter mea/ns lfor limiting Vthe illumination of said light sensitive member in .the other direction in' accordance with the average sound intensity,

said last mentipned means ,including a coil..

for normally moving said shutter lmeans i-I'r,125

'accordance with said sound intensity to move said shutter means in the opposite, direction."

' f 'HUGE ltiolDOWlfLL,JR.'l

one direction, and av second coil actuated'in 

